This invention relates to a non-linear bulk capacitance bootstrapped current switch.
In NMOS and PMOS FETs there are inherently parasitic non-linear capacitances between the bulk and the drain and the source (bulk capacitances). The source to bulk parasitic capacitance has been typically reduced or eliminated either by shorting the bulk to the source or connecting the bulk to an external voltage source higher than the source voltage to reverse bias the capacitance. In conventional current output DAC""s, FET""s are used to construct each cell including a differential current switch and a current source for driving the switch. Thus the common source node of the current switch is subject to the source to bulk capacitance of the current switch and the drain to bulk capacitance of the current source. A differential current switch will exhibit distortion on its output as a result of the non-linear bulk capacitances associated with the common source node. These non-linear bulk capacitances can be a result of just the current switch or can also include any device connected to the common source node. Connecting the bulk to the source of the current switch is not wholly effective because while it reduces the source to bulk capacitance, the drain to bulk capacitance of the current switch is now connected to the common source node. And the bulk to drain capacitance of the current source is still present. Connecting the bulk of the current switch to a fixed voltage greater than the source voltage reduces the non-linearity of the bulk capacitances but the capacitance is still present and still causes frequency roll-off because the capacitance is not fully shorted since the fixed voltage does not precisely follow the source voltage.
It is therefore an object of this invention to provide an improved non-linear bulk capacitance bootstrapped current switch.
It is a further object of this invention to provide such an improved non-linear bulk capacitance bootstrapped current switch with improved distortion performance.
It is a further object of this invention to provide such an improved non-linear bulk capacitance bootstrapped current switch which reduces effective capacitance on the common source node.
It is a further object of this invention to provide such an improved non-linear bulk capacitance bootstrapped current switch which effectively shorts the non-linear component of the capacitances on the common source node.
It is a further object of this invention to provide such an improved non-linear bulk capacitance bootstrapped current switch in which the bulk and source voltage track.
This invention results from the realization that by connecting the bulk connections of one or both of the current switching circuit and current defining circuit to a tracking circuit so that the voltage on the bulk capacitance follows that on the source node of the current switch those non-linear bulk capacitances see no signal voltage over them so the normal roll-off effect of their capacitance as well as their non-linearity is removed.
This invention features a non-linear bulk capacitance bootstrapped current switch including a current switching circuit including a source node and a first bulk connection having a first bulk capacitance associated with it. A current defining circuit is connected to the source node and has a second bulk connection with a second bulk capacitance associated with it. A tracking circuit drives the voltage on at least one of the first and second bulk connections in response to the voltage on the source node to reduce the associated non-linear bulk capacitance. In a preferred embodiment the current switching circuit may include an FET. The current switching circuit may include a differential current switch which includes FETs and the source node may be a common source node. The FETs may be PMOS or NMOS. The first bulk capacitance may be a source to bulk capacitance. The current defining circuit may include a current source; it may include a current sink. The tracking circuit may be connected to both bulk connections. The tracking circuit may include a DC level shifting circuit. The current defining circuit may include a cascode circuit. The second bulk capacitance may be the bulk to drain capacitance of the cascode circuit. The first bulk capacitance may be the source to bulk capacitance and the second bulk capacitance may be the drain to bulk capacitance. The tracking circuit may drive the voltage on at least one of the bulk connections to be equal to the voltage on the source node or to be greater than the voltage on the source node or less than the voltage on the source node depending in part on whether the current defining circuit is a current source or current sink.